McGinty and O Shea and Groundforce solves sewer challenge

01 Sep

Main Contractor McGinty & O’Shea, one of Irelands leading civil engineering & project management about halfway through a two-year sewer upgrade project at Dunmore East, a picturesque fishing village in County Waterford.

The sewer upgrade involves the construction of several underground structures required for the collection and disposal of sewage and storm-water run-off, including a pumping station and an outfall pipe which will discharge surface water out to sea.

This latter structure required the construction of a manhole chamber which when completed would form part of the harbour wall at the strand in Dunmore East.

McGinty O’Shea therefore had to create a three-sided cofferdam, open onto the beach along its fourth side.

Difficult conditions combining loose sandy soil, high ground water table and the effect of the tide made the task of supporting the excavation very complex. The biggest challenge though, was the fact that the top 4m of the 7m-deep excavation was open on the side facing out to sea.

Following initial requirements supplied by project manager Robert Murphy, Groundforce designed a system to help solve this problem, which was then approved and amended by McGinty & O’Shea PSCS, to reach a final solution.

Groundforce’s Joseph Lenihan, explains the solution proposed to McGinty O’Shea. “Normally you transfer the load from one side of an excavation to the opposite side but of course we couldn’t do that with the side facing the opening where the sea wall had been dismantled,”

One alternative in this situation is to use the two perpendicular sides as a ‘shear key’ by welding the frame to the sheet piles. But this wasn’t possible as the shear strength capacity of the soil wasn’t sufficient to ensure stability.

Neither was it possible to increase the size of the shear key by extending the excavation further back from the wall, as existing services were buried beneath the pavement and road.

The solution was found in the form of two 14m3 concrete thrust blocks cast on the beach in front of the excavation to take the lateral loads.

“The thrust blocks were cast just below the level of the beach and the lateral loads were transferred to them from the lower two frames using two 80-tonne hydraulic struts. This provided the lateral restraint and prevented the cofferdam from slipping out onto the beach,” explains Joseph.

Some lightweight interlocking trench sheets were tack-welded to the frames on the open side to prevent fines washing into the excavation at high tide, and with the thrusts blocks buried beneath the sand, there was no visible sign of the support system.

Groundforce supplied its Mega Brace and Maxi Brace hydraulic frames to support the L603 sheet-piles and an Ms4 piling hammer to install and extract the piles, with edge protection and ladder access systems also provided.

Due to the depth of the excavation, the support system comprised four levels with the two top frames welded to the sheet piles to maximise the shear key, with loads from the lower two frames being transferred to the thrust blocks.

McGinty & O’ Shea carried out the installation of the cofferdam with their experienced sheet piling crew. They then constructed the cast-in-situ deep manhole before backfilling, rebuilding the harbour wall and resurfacing the pavement.

Elsewhere on the project, Groundforce also supplied KD6 sheet piles, an MS4 hammer and its Mega Brace system to support an excavation for the new Harbour Pumping Station.

Here, difficult ground conditions also posed a problem. “It was only possible to drive sheets on one of the four sides and a sheet toe-in was not possible because of an outcrop of hard rock,” explains Joseph Lenihan.

The solution here was to employ two levels of Mega Brace frame to support the sheet piles and transfer the loads onto the rock on one side of the excavation and to the mass concrete foundations of a neighbouring building on the opposite side.

Despite the challenging constraints on both excavations all works were completed in a safe, cost effective and timely manner by McGinty & O’ Shea, who has a wealth of experience in carrying out specialist civil engineering work.